Omenn syndrome is a human disease that exhibits an unusual combination of immunodeficiency coupled with autoimmunity. Patients have few mature T cells or B cells, and have poor immune responses. On the other hand, patients show substantial serum antibodies with elevated levels of self-reactive antibodies. Hypomorphic mutations in recombination-activating genes (RAG) are some of the best-characterized genetic defects associated with Omenn syndrome. RAG proteins are critical for recombination of receptor genes during development of B and T lymphocytes and hypomorphic RAG mutations lead to limited generation of mature lymphocytes. However, the causes of autoimmunity in the disease are not clear. During the past several years, our research groups have collaborated on studies of B cell differentiation and tolerance during early B cell development. We have recently shown that isotype switching occurs even in immature B cells in the bone marrow, counter to the prevailing paradigm that switching was limited to activated mature B cells. We have also found that immature B cell switching is greatly elevated in a mouse strain (564Igi strain) that models human System Lupus Erythematosus (SLE), and develops autoimmune disease. Because isotype switched B cells respond more strongly to stimulation, we have hypothesized that isotype switching in immature B cells can lead to a breakdown of central tolerance checkpoints and can result in autoimmune disease. We have also developed mutant 564Igi variants that either can or cannot undergo isotype switching in immature B cells. Preliminary studies indicate that autoimmunity is greatly reduced in 564Igi mice that lack early B-cell isotype switching, supporting our hypothesis that early B-cell switching can circumvent tolerance and can lead to autoimmune disease. The SLE-like 564Igi mouse model effectively reproduces many features of human SLE, but the gene- targeted anti-RNA antibody genes in the 564Igi strain are a genetic construct that is not found in human SLE patients. We want to test the role of early B-cell switching in the development of autoimmunity arising from a genetic mutation known to be involved in human autoimmunity. Recent reports have described mouse models of the human Omenn syndrome disease. These have a gene-targeted hypomorphic RAG gene and, thereby, are analogous to patients with hypomorphic RAG. The mouse models replicate the lymphopenia, the serum antibody levels, and the autoimmunity of the human disease. Furthermore, some features of Omenn syndrome, such as anti-RNA, anti-chromatin and anti-DNA antibodies, as well as elevated serum cytokines that regulate isotype switching, are shared with SLE. We will use Omenn syndrome mouse model variants that cannot undergo isotype switching in immature B cells to test whether these exhibits reduced autoimmunity. Our tests will indicate whether or not tolerance breakdown through isotype switching might be a common feature of various B cell autoimmune diseases.